Photoinduced Charge Transfer in Poly(3-hexylthiophene)/TiO2 Hybrid Inverse Opals: Photonic vs Interfacial Effects

Abstract

We study the effects of tailored light–matter interactions on charge transfer in conjugated polymer films. We use inverse opal structures of titania as the electron acceptor and the model polymer poly(3-hexylthiophene) (P3HT) as the electron donor. We systematically tune the periodicity of the inverse opal to study how the photophysical properties of the conformally coated P3HT are affected by the photonic stop band and band-edge light localization to observe both suppression and enhancement of absorption. Surprisingly, we observe changes in the vibronic coupling in the P3HT absorption spectra in the inverse opal structures as compared to control films. We determine that the polymer in the inverse opals shows more J-aggregate-like behavior with exciton bandwidths of 18 meV, compared to that of 124 meV for P3HT on a planar mesoporous TiO2 film. We also study charge transfer at the polymer/inorganic interface by photoinduced absorption spectroscopy. We find that the polaron signal depends on the excitation wavelength, the periodicity of the inverse opal, and the interfacial area. The inverse opal structures exhibit significantly increased charge generation compared to the control films, and we determine that photonic effects of the lattice, while observable, play a secondary role in this enhancement relative to the increased surface area.